1. Field of the Invention
The present invention relates to a field of image encoding.
2. Description of the Related Art
In a field of image encoding, there have been already proposed several techniques by which an edge (edge portion) of an object in an image is detected, and an image encoding control is executed on the basis of the detection result (edge information) (refer to Patent documents 1 to 5, for example).
There arises such a problem that, in an edge that forms a boundary of a complicated portion (large in change) and an edge that forms a boundary of a flat portion (small in change) in an image, the deterioration of the image in the edge that forms the boundary of the flat portion is more visible than that of the boundary of the complicated portion. It is desired to prevent the deterioration of the image quality by detecting such edges and conducting an image quality improving processing that is high in precision.
For example, in a method of detecting an edge by determining a difference of pixel values as disclosed in Patent documents 1 to 5, minute edges in pixel unit (for example, patterns of an object) are detected, and the method is not suitable for conducting the image improving process.
That is, because there are few influences made by the minute edges on human eyes (vision), even if the image improving processing is executed, the improvement effect is low. On the other hand, in the image encoding, a data (information) size of an image after being encoded is predetermined. Therefore, if a number of edges are detected, since data (information) amount for an image quality improvement which can be allocated to one edge is smaller, it is caused a possibility that the image quality improving processing cannot be conducted, sufficiently.
As edge detecting methods, there exist techniques that are applied in an image recognizing field other than the techniques disclosed in the above-mentioned Patent documents 1 to 5. However, it is difficult to apply the edge detecting methods in the image recognizing field to the image encoding field that requires real-time operations because the edge detecting methods in the image recognizing field are complicated and voluminous as a whole.
[Patent document 1] JP 9-167240 A
[Patent document 2] JP 2001-14476 A
[Patent document 3] JP 9-120458 A
[Patent document 4] JP 2003-230147 A
[Patent document 5] JP 8-194824 A